1. Field of the Invention
The present invention relates to a mixing device which comprises a shaft (1), a front ring (2), which is positively and non-positively connected to this shaft, an end ring (3), which is positively and non-positively connected to the shaft at a distance A from the front ring, and a loose mixing ring (4), which is freely rotatable and can be moved back and forth between the front ring and the end ring.
2. Description of Related Art
Devices for mixing polymer melts are known from the prior art.
For instance, DE-A 100 00 938 discloses a mixing device in which an assembly of mixing rings and dividing rings joined, alternately one behind the other has been pulled onto a rotor. The mixing rings have grooves, the dividing rings have bores. If melt passes over from the mixing grooves to the bores, it is deflected and divided and is mixed in this way. A similar principle is disclosed in EP-A1 1 000 656, with the difference that some of the rings of the ring assembly are freely rotatable. Both devices have the disadvantage that, on the one hand, they are structurally complex, susceptible to wear and difficult to clean and, on the other hand, high pressures are required to force the melt through the bores.
EP-A 48590 discloses an extruder mixer comprising a rotor and a stator, both the rotor and the stator having rows of semicircular cavities. The cavities of the rotor and stator do not lie one over the other but are offset somewhat with respect to one another. As a result, the extruded material is not only sheared but also divided and rotated. This device too is structurally complex and, furthermore, has the disadvantage that the mixing effect is only sufficient for practical requirements with relatively long stators. In addition, when these stators are used in an injection-molding machine, a non-return valve must be additionally fitted, since the mixing device itself is not suitable for preventing the melt from flowing back.
EP-B1 219 334 relates to a cavity transfer mixing extruder which comprises a stator and a rotor and is distinguished by the fact that both the stator and the rotor have two cavity groups. These cavity groups are aligned with each other in the form of a parallelogram when they are projected onto a plane and, furthermore, are characterized by their angles of inclination. This mixing device is disadvantageous because, like the device referred to in EP-A 48590, this device is expensive to produce and its mixing effect depends on the length of the stator.
JP 50-90117 discloses a mixing device for injection-molding machines which operates in the manner of a non-return valve. The mixing device in this case comprises two rings, one being screwed onto the screw shaft and the other being arranged movably over it. Both parts have channels, which are arranged in such a way that they overlap and the melt is transferred from one channel directly into the other. The mixing effect of this device is not adequate for many applications.
In the patent EP-B1 340 B73, a mixing device for an extruder or an injection-molding machine is described. This device has a separate mixing ring, arranged for free rotation about a rotor. The rotor is arranged in a stator. The mixing ring is distinguished by the fact that it has mixing passages, which extend from the inner surface of the stator to the outer surface of the rotor. In other words, the mixing ring has holes. The mixing device according to EP-B1 340 B73 can assume the function of a non-return valve for injection-molding machines. The mixing device disclosed in the patent has the disadvantage that the melt transport is not ensured sufficiently well for all applications. On the other hand, the mixing device does not provide an adequately good seal when it is used as a non-return valve. In addition, the mixing effect is only at its optimum when the mixing device has a certain minimum length. The minimum length is about twice the screw diameter 2D. Therefore, this mixing device cannot be adapted to every extruder or every injection-molding machine, but instead the machines must be specifically converted for this purpose, that is to say the screw must be shortened. Since the mixing device must have a certain minimum length, the melt volume that is constantly present in the mixing device is also relatively great. This means that, for example when changing colors, relatively considerable time and material is required before usable parts of the new color are obtained. On account of the fact that the mixing ring is provided with holes, it also has the disadvantage that it is mechanically weakened. The shorter the ring, the more acute this disadvantage, making it highly susceptible to wear.